powerpc/pmac: remove unused symbol

sccdbg symbol is not used, so remove it

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zucheng Zheng <zhengzucheng@huawei.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210409093519.118000-1-zhengzucheng@huawei.com